Polymers having narrow molecular weight distribution have characteristics of having low viscosity compared to polymers having broad molecular weight distribution with the same number-average molecular weight. Block copolymers, compared to random copolymers, retain physical and chemical characteristics carried by each block, and for example water soluble-nonwater soluble diblock copolymers, similarly to low molecular weight emulsifying agents, have characteristics of forming micelles in an aqueous solution with the water soluble block facing the aqueous phase and the nonwater soluble block as the core. In order to obtain polymers and block copolymers having narrow molecular weight distribution, a sophisticated function of controlling polymerization is required.
As a polymerization method having a sophisticated function of controlling polymerization for obtaining a polymer and a block copolymer having narrow molecular weight distribution, living radical polymerization (sometimes referred to as “controlled radical polymerization”) is known. Depending on the mechanism of polymerization, several types of polymerization methods are known. Among them, a polymerization mechanism in which chain transfer during polymerization proceeds reversibly is useful as a polymerization method to obtain a polymer and a block copolymer having narrow molecular weight distribution. As such a polymerization method, reversible addition-fragmentation chain transfer (hereinafter referred to as “RAFT”) polymerization has been proposed.
As a method of producing polymers, from the viewpoint of industrial production, emulsion polymerization is superior in terms of removing reaction heat and recovering polymers, and thus techniques of carrying out controlled radical polymerization using emulsion polymerization may be desired. Various investigations have been made on emulsion polymerization that employs “RAFT agent,” a polymerization controlling agent that permits RAFT polymerization. In emulsion polymerization, however, an emulsifying agent in addition to a RAFT agent must generally be added, which in many cases may markedly reduce polymerization speed or reduce latex stability. Thus, in emulsion polymerization that requires the addition of an emulsifying agent in addition to the RAFT agent, it is known that its ability of controlling molecular weight is poorer than the homogeneous solution polymerization, and thus there is a need for a RAFT agent that acts not only as an emulsifying agent but also as a polymerization controlling agent.
Patent Document 1 illustrates a compound that serves as both an emulsifying agent and a polymerization initiator. However, the molecular weight distribution shown in the Examples of Patent Document 1 has a very broad range, and thus it cannot be recognized to be superior to molecular weight distribution in an emulsion polymerization that does not use controlled polymerization.
In Patent Document 2 and Non-patent documents 1 and 2, RAFT agents having introduced therein polyethylene glycol (PEG) units having a surface-activating ability are reported. However, in polymerization using this RAFT agent, PEG units may inevitably be introduced into the ends of the polymer obtained, and though sulfur-containing units (derived from a RAFT agent; they are dithioester sites in many cases) that cause coloration can be removed during the post-treatment step using a radical-generating agent such as an azo compound and a peroxide, or an amine, it is difficult to remove PEG units as well that are sites having a surface-activating ability (hereinafter referred to as “surface-active sites”). As a result, polymers containing residual PEG units may pose a problem of water absorptivity depending on the intended uses.